Regarding low-molecular-weight organic electroluminescence (hereinafter abbreviated as OLED) elements, it has been reported that when a copper phthalocyanine (CuPC) layer is provided as a hole-injecting layer, a lowering of drive voltage, an improvement in initial characteristics such as an emission efficiency, and also an improvement in lifetime characteristic can be achieved (Non-patent Document 1: Applied Physics Letters, USA, 1996, Vol. 69, pp. 2160-2162).
Regarding organic electroluminescence elements using polymer light-emitting materials (hereinafter abbreviated as PLED), on the other hand, it has also been reported that thin films of polyaniline materials (Patent Document 1: JP-A 3-273087, Non-patent Document 2: Nature, Britain, 1992, Vol. 357, pp. 477-479) or polythiophene materials (Non-patent Document 3: Applied Physics Letters, U.S.A., 1998, Vol. 72, pp. 2660-2662) are used to obtain a similar effect as with OLED elements.
In recent years, there has been found charge-transporting varnishes made of homogeneous solutions wherein highly soluble low-molecular-weight oligoaniline materials or oligothiophene materials are used and completely dissolved in organic solvents. It has been reported that when the hole injecting layer obtained from this varnish is inserted into an organic electroluminescence (hereinafter referred to as organic EL) element, there can be obtained a flattening effect of an underlying substrate and excellent EL element characteristics (Patent Document 2: JP-A 2002-151272 and Patent Document 3: WO 2005/043962).
The low-molecular-weight oligomer compounds per se are low in viscosity. When ordinary organic solvents are used, a process margin in film-forming operations is so narrow that when using a variety of coating methods such as spin coating, ink-jet coating and spray coating and a variety of baking conditions, a difficulty has been involved in homogeneous film formation.
However, the use of various types of additive solvents enables the viscosity, boiling point and vapor pressure to be controlled, which, in turn, enables a highly homogenous film-formed surface to be obtained according to the various coating methods (Patent Document 4: WO 2004/043117 and Patent Document 5: WO 2005/107335).
Hence, low-molecular-weight oligomer compounds have been recently used as a hole-injecting layer in organic EL elements.
However, there are demands of a further improvement in solubility of low-molecular-weight compounds sufficient to permit easy matching with a variety of coating methods such as spin coating, ink-jet coating and spray coating, and also of a further improvement in conductivity and emission efficiency and luminance characteristics when applied to organic EL elements.
Especially, improvements of an emission efficiency and luminance characteristics are important for both OLED elements and PLED elements.
Characteristics of a hole-injecting layer include hole injectability into an emission layer, electron blocking capability of from the emission layer, and capability to inhibit deactivation of excitons in the emission layer. These functions greatly influence the emission efficiency and luminance characteristics of such organic EL elements as set forth above. Thus, there is a demand of low-molecular-weight oligomer compounds that are able to realize an excellent function as a hole-injecting layer.
It has been reported that when silane compounds are added to compositions including conductive polymers such as polystyrenesulfonic acid or polyaniline, the lifetime of organic EL elements having a thin film formed from this composition is prolonged (Patent Document 6: JP-A 2003-45667). Nevertheless, no case of adding a silane compound to charge-transporting varnishes including low-molecular-weight oligomers has been reported.